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Be sure to have the Home tab selected in the ribbon and click on the i icon on the Metadata button.The Image Metadata dialog box will be displayed.Information is displayed for a default data layer (layer#1,i.e.,band #1). You can change the layer number to get the information for a different data layer if your data is multiband data, despite what the image is being displayed as in the viewer.It shows the spread and shape of the how the pixel values are distributed over all of the possible values for that particular band. The x-axis represents the digital number and the y-axis shows how many pixels in the image have that particular digital number. Explore the layers of the image and use the histogram to see how the digital number values of one band might or might not overlap with another band.Regarding comparison you may open two views at the same time and do the same above mentioned and see the differences.

thanks for the hint! I knew it must be somewhere close, but could just not see the button! do you know by any chance how i can make the histograms appear even when I have a lot of NAs? i am now in the image metadata and can see all the buttons and also use almost all of them. only the histogram stays blanc... and i have a lot of NA values...
– Li12Aug 21 '18 at 15:15